1. Field of the Invention
The present invention relates to devices and techniques for non-contact thermal treatment of a patient's cornea for altering anterior corneal curvature to correct refractive disorders, and more particularly, to implantable intrastromal segments that may be elevated in temperature by magnetic induction to cause lesions in, or shrinkage of, the Bowman's layer and/or stromal lamellae to increase corneal steepness in hyperopic or presbyopic refractive treatments.
2. Description of the Related Art
Refractive disorders of the eye result from the inability of the eye's optic system, consisting of the dome-shaped cornea and the crystalline lens just behind it, to properly focus images on the retina, the nerve layer at the back of the eye. Approximately 80 percent of the refracting power of a human eye is within the cornea. When the cornea is mis-shaped, or the eye is too long or too short along its optical axis, or when the lens of the eye does not function normally, a refractive error occurs. Refractive errors generally include hyperopia, myopia, presbyopia and astigmatisms. Hyperopia is a refractive error that causes poor close-up vision, and is caused by a flattened cornea or by a shortened eye that focuses images beyond the retina (see FIG. 1A). Myopia is the opposite and causes poor distance vision, and is characterized by an elongate eye or steepened corneal shape. This condition causes distant images to focus in front of the retina rather than directly on it (see FIG. 1B). Presbyopia results from aging and is a form of farsightedness caused by diminished ability of the lens to elastically change to refract light. Astigmatism is a condition which causes blurred vision for both near and far objects. In an astigmatic patient, the cornea may be shaped like the back of a spoon rather than having a spherical shape. Such an asymmetric corneal shape creates different retinal focal points. Hence, instead of images focusing on the retina, the images focus on a number of points around the retina resulting in a blurred image.
The optimal shape for a cornea is that of a perfect sphere assuming that axis of the eye is normal relative to the other eye. Glasses and contact lenses correct refractive errors by refracting (bending) light before it reaches the cornea and is transmitted through the lens, in other words, changing the angle at which light enters the cornea.
Several types of surgical procedures have been developed to correct refractive disorders such as myopia, hyperopia and astigmatisms by changing the shape of the cornea. For example, laser procedures can reshape the patient's cornea to some extent to a corrected more spherical shape, the most common procedures being laser in-situ keratomileusis (LASIK) and laser photorefractive keratectomy (PRK). LASIK and PRK correct vision by recontouring the anterior layers of the cornea by means of surface ablation with a laser.
It is useful to provide a description of the anatomy of the patient's eye. FIG. 1C depicts patient's eye 5 which comprises a system of cornea 6 and lens 3 which focuses light on the retina indicated at 4 which is at the back of the substantially spherical body defined by sclera 7. The anterior chamber 8 (and aqueous 9a therein) is separated from the vitreous body 9b by lens 3. Thus, cornea 6 forms the anterior wall of chamber 8 and also acts as a lens element. The cornea 6 is a smoothly curved transparent structure which has a smaller radius of curvature than the opaque sclera 7 and bulges from the smooth outer spherical surface of the eye. Refractive errors occur when the cornea and lens do not focus incoming light on retina 4.
The cornea 6 is uniquely structured to transmit light into the eye. The primary structure of the cornea is the stroma, which comprises approximately 90 per cent of the cornea's thickness. The stroma is comprised of lamellae which lie in flat sheets and extend from limbus to limbus. Each lamella (ayer or sheet) consists of strong, parallel collagen fibrils which are maintained in a regularly spaced hexagonal separation by a ground substance or GAGs (for glycoaminoglucans, also called a glycoprotein and mucopolysaccharide matrix). Between the lamellae are keratocytes layers (the fibroblasts), the constitutive cells of the cornea which produce the GAGs and support synthesis of collagen. It is well known that the elongate stromal collagen fibrils may be longitudinally contracted by application of heat to temperatures above about 60.degree. to 65.degree. C. See, e.g., U.S. Pat. No. 4,461,294 issued to Baron; U.S. Pat. No. 4,976,709 issued to Sand; U.S. Pat. Nos. 4,326,529 and 4,381,007 issued to Doss; and U.S. Pat. No. 5,533,999 issued to Hood.